How Does Rain Affect Road
Accidents ?
By Ann Nguyen
19/August/2016
The cost of road accidents
• Globally: 3000+ fatal road accidents per day
• Australia: 1,200 people died and 34,000 people were hospitalised each
year. Costs $27 billion
• South Australia:
 2015: 102 Fatalities, 759 serious injuries
 Fatal accident rate (per 100,000 pop) of 6.4 (2014), 6.0 (2015) compared
to the national average rate of 4.92 (2014).
 Costs $1 billion each year in SA community
• The long lasting cost of road accidents: The suffering of victims, the loss
of loved ones.
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The cause of road accidents
The consequence of the combined effects (variously
involved ) :
• Behavioural: Alcohol and/or other drugs, speed,
driver fatigue, dangerous driving behaviours
• Technological: Vehicle malfunction
• Environmental factors: Road conditions, adverse
weather
Environmental Factor
• This risk to driving in the rain is called “low skid
resistance
• A coefficient friction between the tire and pavement in
a dry road (0.5), a wet road (0.3).
• Wet weather requires a greater braking distance and
stopping distance
• E.g.: The speed of 88km/h requires a stopping distance
of 131 meters (wet road), 96 meters (dry road).
2
Visibility: Decreases as much 74% in wet
weather depending on speed travels
E.g.: In clear visual environment, a detection
distance of 208m, but 54m (adverse weather)
Background
• Rain increases of 75% of traffic collisions and
45% increases of injuries in Canada (Andrey,
Mills et al. 2003)
• Rain causes of 7,000 highway deaths and
800,000 injuries in U.S (AMS 2016)
• Rain causes 13.8% of freight road accidents in
Great Britain (Jaroszweski 2012)
3
How do we determine the risk rate of road
accidents in wet weather?
•
The matched pair method: EVENT (wet), CONTROL (dry):
E.g.: Wet Accidents occur at 9-11am on Monday matches Dry
Accidents occur at 9-11am on Monday in the preceding or
following week
A: Number of road accidents in the EVENT periods
B: Number of road accidents in the CONTROL periods
Other studies:
A
RRR = --B
At per 30 minutes:
A1: Number of road accidents in the wet weather condition
B1: Number of road accidents in the dry weather condition
C: Number of wet periods (The frequency of wet periods)
D: Number of dry periods. (The frequency of dry periods)
•
Our study:
A1/C
RRR = --------B1/D
Rain Threshold:
Dry weather condition: < 0.2mm
Wet weather condition: >= 0.2 mm
The Relative Risk Ratio
• RRR = 1: There is no difference between the
risk of road accidents on wet vs. dry condition.
• RRR > 1: Increased risk in wet condition.
• RRR < 1: Decreased risk in wet condition.
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The RRR from Previous Study
Study
Location
Period
Time
Observation
RRR
Andrey et al. (2003),
Canada
6 mid-sized cities
1995-1998
6 Hours
1.73
1984-2002
6 Hours
1.5
10 Canadian cities
Andrey (2010), Canada
Keay& Simmonds (2005),
Australia
Melbourne city
1989-1996
3 Hours
1.61-1.67
Dai (2011), U.S
Oregon 217 freeway,
Portland
2007
2 Hours
1.36
Hambly et al. (2013),
Canada
Greater Vancouver
2003-2007
Daily
1– 2
Jaroszweski & McNamara
(2013), UK
Manchester city
2008-2011
3 Hours
1.5
London city
Nguyen & Ostendorf
(2016)
South Australia
1.18
1995 - 2013
30 minutes
1-3
Study Area
Appx. 20 million of half-hourly rainfall observations from a network of 60 rain gauges in the
temporal scale of 18 years
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Location of Road Accidents
Appx. half a million time-space referenced road accidents at a broad spatial scale of a million
square km
Our findings
Adelaide region has an increase of the relative risk of road accidents of 70% (RRR =1.7).
While outback areas has 200% increase in risk of road accidents (RRR = 3) in wet
weather in compared to dry weather conditions..
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The RRR of Road Accidents in the Day
The RRR of Road Accidents in the
Week
2.5
RRR
2
1.5
1
0.5
0
Sunday
Saturday
Monday
Tuesday
Wednesday
Thursday
Friday
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The RRR of Road Accidents by Months
2.5
Relative Risk Ratio
2
1.5
1
0.5
0
1
2
3
4
5
6
7
8
9
10
11
12
Month
High Risk of Road Accident
Types in Wet Weather
Number of Accident Types
1468
Right Turn
23986
Right
Turn
1888
Side Swipe
1.95
35141
Side
Swipe
1.71
3256
Hit Fixed Object
Hit
Fixed
Object
42846
4325
Right Angle
Right
Angle
64961
7420
Rear End
109653
0
20000
40000
60000
80000 100000
Acc in Wet Weather
Acc in Dry Weather
2.42
120000
2.12
Rear
End
2.15
RRR
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Summary
Advantages:
The study conducts in a large spatial and temporal scale of SA in 18 years.
Rain affects road accidents variously in SA. The relative risk of road accidents increases in Adelaide
region (70%), outback areas (200%). The risk rate of Adelaide city is similar to Melbourne city (67%)
Most dangerous times to drive:
• The work traffic periods of 7:30–9:30am and the evening rush hours of 17:30–18:30pm (90% –
157%).
• Tuesday (119%)
• January and May (135% & 145% ).
Most dangerous road accident types:
• Hit fixed object (142%), rear end (115%), right angle (112%)
Disadvantages:
Potential error of data: The reset time of rain gauges at 9am each day.
The limited coverage of rain gauges
Recommendation:
weather radars could address larger spatial coverages of rainfall variety with a shorter observation
References
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AMS (2016). "Weather and Highway Safety“. Retrieved 14 August, 2016, from
https://www.ametsoc.org/ams/index.cfm/policy/studies-analysis/weather-and-highway-safety/
Andrey, J., et al. (2003). "Weather as a chronic hazard for road transportation in Canadian cities."
Natural Hazards28(2-3): 319-343.
BITRE (2015). "Yearbook 2015- Australian Infrastructure Statistics." Retrieved 25 April, 2016, from
https://bitre.gov.au/publications/2015/yearbook_2015.aspx.
DPTI (2011). "South Australia's Road Safety Strategy 2020." Retrieved 25 March, 2015, from
http://dpti.sa.gov.au/__data/assets/pdf_file/0020/82163/South_Australias_Road_Safety_Strategy_
to_2020.pdf.
Jaroszweski, D. (2012). The Impacts Of Climate Change On The National Freight Sector Transport
and climate change: a review. L. Chapman, Ryley, T. . 2: 142.
SAPOL (2016). “Accident Statistics”. Retrieved 25 April, 2016, from 14/08/2016
https://www.police.sa.gov.au/about-us/traffic-statistics
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QUESTIONS
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